Customer says my chemical treatment damaged their pool. Anyone dealt with this?

So I had a customer call me 4 days after a chem treatment claiming that I caused some discoloration in the pool. But the thing is, the pool alreadty had stains when I showed up. I even mentioned it at the start (but I didn't take photos, which in hindsight, was a stupid decision)

Now he's talking about repair costs and arguing that I caused the damage. How are other pool contractors protecting themselves from situations like this? Do you document every job or is dealing with these type of owners just part of the busines?